[FIX] mrp: workorders of sub-BOMs

- Create the following BOM structure:
```
  Prod 1 (Manufacture)
  |
  --- Prod 2
      Prod 3 (Kit)
      |
      --- Prod 4
	  Prod 5
```
- The BOMs for Prod 1 & 3 must have a routing set, to generate
  workorders
- Create a MO for 1 unit of Prod 1, process to the end

2 units of Prod 1 are created instead of 1.

This is due to the WO for Prod 3 not having a `next_work_order_id`.
Therefore, `record_production` adds an extra quantity at:
```
production_move.quantity_done += self.qty_producing
```

To prevent this, we set as `next_work_order_id` of the BOM of Prod 3 the
WO of Prod 1.

Closes odoo#19527
opw-1817398
opw-1883693

addons/mrp/models/mrp_production.py

@@ -435,10 +435,16 @@ def button_plan(self):
     @api.multi
     def _generate_workorders(self, exploded_boms):
         workorders = self.env['mrp.workorder']
+        original_one = False
         for bom, bom_data in exploded_boms:
             # If the routing of the parent BoM and phantom BoM are the same, don't recreate work orders, but use one master routing
             if bom.routing_id.id and (not bom_data['parent_line'] or bom_data['parent_line'].bom_id.routing_id.id != bom.routing_id.id):
-                workorders += self._workorders_create(bom, bom_data)
+                temp_workorders = self._workorders_create(bom, bom_data)
+                workorders += temp_workorders
+                if temp_workorders: # In order to avoid two "ending work orders"
+                    if original_one:
+                        temp_workorders[-1].next_work_order_id = original_one
+                    original_one = temp_workorders[0]
         return workorders
 
     def _workorders_create(self, bom, bom_data):